Berlin is the city of vans, and Volkspark Hasenheide is definitely one of the hotspots. Right in the park, a tiny dead end road and lots of old campers with friendly people around. Some have been living here in their vans from as early as 1987. Be aware that police is tolerating camping, but you might get a ticket of 80 Euros if your van does not have the green particulate matter sticker (Feinstaubplakette). Police is by far not checking every day, but once in a while (you are not allowed to enter the city without the green sticker anyway).